1. Field of the Invention
The present invention relates to a light source system, an optical scanner, an image forming apparatus, and a light-amount control method.
2. Description of the Related Art
A raster output scanner having a light source such as a surface-emitting laser to emit a number of light beams has been proposed. For example, Japanese Patent Application Laid-open No. H10-100476 discloses a raster scanner that uses a semitransparent and semi-reflecting member as a photo-detector to reflect light emitted from the light source.
Besides, for example, Japanese Patent Application Laid-open No. 2002-26445 discloses a light source device of a type using a surface-emitting semiconductor laser, having a light-receiving surface placed on a substrate of the semiconductor laser. The device detects an amount of emitted light with a splitter that splits a traveling path of light beams emitted from the semiconductor laser into a plurality of paths so that the light beams pass through the light-receiving surface.
Japanese Patent Application Laid-open No. 2005-274678 discloses an optical scanner having a plurality of light emitting points, an optical unit that collimates light beams, and a beam reflecting member that transmits a portion of the light beams therethrough and reflects another portion of the light beams therefrom. The optical scanner detects a light amount pertaining to even a light source that has no back beams. In the optical scanner, a unit that detects the light amount and the light source are disposed on a single base member to miniaturize the optical scanner.
Japanese Patent Application Laid-open No. H06-164070 discloses a light source device provided with an aperture for shaping light beams emitted from a semiconductor laser having a plurality of linearly-aligned light emitting sources, and a plurality of light receiving elements corresponding to the light emitting sources, which receive detecting light beams reflected from a mirror surface provided on the aperture.
Japanese Patent No. 3227226 discloses a technology related to a positional relationship between a scanning line provided by a surface emitting laser and an image-forming spot to thereby attain high-speed scanning.
In recent years, there are increasing demands for an image forming apparatus of higher speed, higher density, and higher image quality. In response to the demands, an image forming apparatus that uses a multi-beam image-forming system having a plurality of light emitting points to scan a single scanning surface with a plurality of beams has been put into practice.
Optical scanners, particularly laser scanners employing a semiconductor laser are advantageous in simple structure, high speed performance, and high resolution, and are therefore in wide use as image forming apparatuses, such as an image display apparatus and an image recording apparatus. Particularly, the optical scanner is suitably used for an exposure device of an electrophotographic printer, and many products are currently on the market as laser printers.
Meanwhile, in recent years, demands for a higher image scanning speed and higher resolution of an image forming apparatus have been increased, and an increase of a scanning speed has been desired. While a high-speed deflector is necessary to attain high-speed scanning, when, for example, a rotary polygon mirror is employed as the deflector, a problem that the rotation velocity has an upper limit arises.
As a solution for this problem, Japanese Patent No. 3227226 proposes an image forming apparatus that uses a so-called multi-beam scanner. The scanner scans with light beams emitted from a vertical cavity surface emitting laser (VCSEL) having a plurality of independently-controllable light emitting points, and performs scanning with a plurality of scanning lines simultaneously in a single scanning operation.
FIG. 1 is an example of a conventional optical scanner using a VCSEL having such a plurality of light emitting areas.
A light source 101 has a plurality of light emitting areas and a light emission controller for controlling the light emitting areas. A plurality of laser beams are emitted from the light source 101, and coupled to be shaped through a coupling optical unit formed with a coupling lens 102 and a cylindrical lens 103. The thus-coupled beams are deflectively reflected from a deflector 105, which is a rotary polygon mirror, and subjected to scanning in a main scanning direction. Hence, the beams forms a scanning line 109 on a scanning surface (photosensitive member) 108, which is an image surface, through an image-forming optical unit formed with a first scanning lens (106a) and a second scanning lens (106b). In this example, a cylindrical image carrier supporting a photosensitive medium is used as the scanning surface 108.
The image carrier rotates about an axis of the cylinder shown in FIG. 1 to move the image surface in the direction perpendicular to the main scanning direction, thereby performing optical scanning to form an image.
To achieve image forming at higher speed, utilization of multiple beams by employing the VCSEL or the like is conceivable. It is particularly common for a high-speed image-output apparatus to adopt a multi-beam light source for image forming.
When “n” laser beams are simultaneously used, an area on which a latent image is formed is increased by a factor of “n” as compared to that with a single laser beam, and a necessary period of time for image forming is decreased by the same factor “n”. A problem with performing optical scanning and image forming using a plurality of light beams is how to cause the light beams to have little difference in light intensity. This is because, when each light beam has different light intensity, densities of scanning lines differ from a scanning line to another, which results in degradation in image quality. To this end, each of Japanese Patent Application Laid-open Nos. H10-100476, 2002-26445, and 2005-274678 describes a light source or an optical scanner using a VCSEL, which detects a light intensity of each light beam with a light receiving surface and controls light emitting devices with a control circuit based on data on the detected value to attain a desired optical intensity.
To achieve image forming at higher speed, the laser is required to have an increased output power in addition to adopting the multi-beam method. The reason for this requirement is as follows. In an electrophotographic image-forming apparatus, an exposure energy exceeding a required amount per unit area and unit time must be provided on a photosensitive member, which functions as the scanning surface. Accordingly, a greater optical power or an increase in efficiency for light utilization is required to increase a linear velocity (moving speed of the surface of the photosensitive member) of the photosensitive member to thereby achieve a higher speed. While attempts to increase an optical power have been made through development of VCSEL elements, a ratio of the increase achieved so far is small. Hence, it is essential to increase the light utilization efficiency of a light source apparatus, a light source system, and an optical scanner to perform image forming at high density and high speed.
Meanwhile, Japanese Patent Application Laid-open Nos. H10-100476, 2002-26445, 2005-274678, and H06-164070 do not or hardly suggest enhancement of efficiency for light utilization.
With the technology described in Japanese Patent Application Laid-open No. H10-100476, if a so-called half mirror is used, a loss entailed in supplying light to the outside is large.
According to the technology described in Japanese Patent Application Laid-open No. 2002-26445, a light receiving surface for detecting the light amount is mounted in a light source apparatus. Hence, the optical path between the light source and the light receiving surface is reduced in length, which allows highly accurate detection of the light amount and image forming of high quality. However, a triangular prism or the like employed as the splitter produces a great loss of light in supplying light to the outside. Increasing the optical output power is the currently imposed requirement on the surface emitting laser. However, a solution for the requirement has not been disclosed yet.
According to the technology described in Japanese Patent Application Laid-open No. 2005-274678, optical scanning and image forming are carried out using a portion of light beams having passed through a beam returning member. Therefore, when a surface emitting laser, of which optical output power are to be increased, is employed as the light source, a loss in light amount exceeds a negligible amount and hence poses a problem to increase the speed.
The technology described in Japanese Patent Application Laid-open No. H06-164070 does not clarify positional relationships among a mirror surface, an aperture member, and light beams to be subjected to splitting for detecting a light amount. Hence, a great amount of light is to be lost, which is a problem for forming an image at high speed. In addition, provision of a plurality of light receiving elements is a problem for cost reduction of the apparatus.